Pulverized coal is washed by forming a water slurry and dewatering the slurry by passage through a fine coal centrifugal scroll screen machine. A typical machine is known as a vertical fine coal centrifugal scroll screen machine and includes a housing, a screen assembly mounted for rotation on a vertical axis in the housing and comprising a base-of-spoke piece supporting the lower, wider end of a frustoconical screen member, the base of spoke piece cooperating with a dividing wall in the housing to form housing portions receiving filtrate passing through the screen member and product passing through the base-of-spoke piece respectively.
The screened coal product is actively stripped from the screening surface of the screen member by a scroll assembly of helical scroll vanes mounted for differential rotation within the screen member, the edges of which scrape the deposited fine coal and urge it through apertures in the base-of-spoke piece. A drive assembly generally comprising an electric motor and gearbox differentially drives the rotation of the screen assembly and scroll assembly mounted on concentric drive shafts. Incoming fine coal slurry passes down vertically under a gravity head into an inlet assembly supported on said housing and feeding in to the upper, narrow end of the screen member.
The commercially important features of a fine coal centrifuge are throughput, ease of maintenance and reduction of downtime. Prior art fine coal centrifuges may require choking or tuning of the gravity head, limiting throughput.